Trusses that provide structural support for a sloped roof of a building are well known. Traditionally, such trusses have been fabricated on-site, but it is also known to construct a truss in a factory and transport the prefabricated truss to the building site.
A disadvantage of off-site truss fabrication is the problem of storing and transporting such a bulky item. A truss is often designed to span the width of a building and so can be fifteen feet to twenty-five feet or more in length and have a height of three to five feet or more.
Metal building trusses are known, two previous designs being described in U.S. Pat. Nos. 4,435,940 issued to Jeanne Davenport et al. on Mar. 13, 1994, and 4,982,545 issued to Gustav Stromback on Jan. 8, 1991. The specifications of these two patents are incorporated herein by reference.
In a general sense, metal trusses might be preferred over wood in order to avoid a negative impact on forests. More particularly, Davenport et al., have found that a metal truss can be light weight in comparison to a wood truss having comparable load and size specifications. Of course, the overall cost of a truss, including manufacturing costs, shipping, ease of assembly and installation, labor costs, etc., must all be taken into account when evaluating the effectiveness of the truss.
Davenport et al. describe a roof truss having chord members of generally "U"-shaped cross-section. There is a horizontal bottom chord member oriented with the "U" in the upright position and two top chord members having an inverted "U". A truss having a king post and diagonal members of "C"-shaped cross-section connecting the top and bottom chords is shown. For assembly, a length of bottom chord material is cut and positioned atop metal caps to span supporting wall studs and secured to the caps by suitable fasteners, such as screws or nails, depending upon the nature of the caps. The top chords are assembled onto the bottom chord, the legs of the "U" of the bottom chord being received within the channel of the "U" of the top chords and the chords are affixed together by screws, or adhesive or welding. The ends of the top chords at the apex of the truss are affixed together by a plate which straddles the butted ends, again by screws, or adhesive or welding. The top chords extend beyond the ends of the bottom chord to create eaves. Diagonal members are eventually assembled and affixed within the truss structure. A pair of elongate ridge caps are affixed to run between truss apices along the roof peak.
Stromback describes a steel roof truss, developed more recently, which can be assembled on-site. Stromback also shows "U"-shaped chords, which in this case are of roll-formed metal. Each of the chords shown has lengthwise reinforcing ridges formed in the legs and bottom of the "U". Each top chord has a rolled-out flange at the free end of each leg of the "U". Each bottom chord has a rolled flat reinforcing flange at the free end of each leg of the "U". For assembly, the legs of the bottom chord are slipped into the bight of the "U" of the top chord and self-tapping screws are installed in abutting legs for fastening. Web members of "C"-shaped cross-section are installed to run between the top and bottom chords. Shorter vertical segments having a "C"-shaped cross-section are also installed and fastened by self-tapping screws between the top and bottom chords so as to form a load bearing joint located above the wall studs upon which the truss is installed.